In using static sieves, sieve bends or screens formed of laterally spaced wedge-shaped screen wires backed by and welded to normally or right angularly related laterally spaced tie rods, for deliquifying and/or classifying slurries or other solid-liquid mixtures, it is common practice, as in Leeman U.S. Pat. No. 3,344,919, to mount a flat or concave sieve, screen or sieve bend in a housing or holder having counterpart ends and reversible end-for-end for alternating the ends of the sieve bend presented for feeding. It also is common practice, as in the U.S. Pat. Nos. 3,363,769 to Wilmont and 3,452,876, to Ginaven, have the sieve or screen slope downwardly from its feed end and to feed the mixture as a sheet tangentially onto the sieve from an overlying feed box through a discharge opening extending the width of the sieve. In some prior assemblies provision is made for adjusting the area of the discharge opening. Thus, there is automatic adjustment in Wilmot to accommodate oversize particles, while Ginaven controls the rate of feed to the sieve by a vertically shiftable control plate. The present invention is primarily concerned with an improvement in adjustable discharge openings of feed boxes.